Motion Control System of MAX IV Laboratory Soft X-ray Beamlines

At MAX IV we make synchrotron radiation, which is similar to ordinary sun light or dentist X-rays, depending on the needs of the different experiments. The light source at MAX IV is however millions of times stronger than our sun. The light is created in the beginning of a long and straight tube. The tube, called a beamline, is packed with equipment to monitor the position of the light, shape the size of the light, select particular energies i.e. colours from the light, and at the end of the tube, make the light shine on a sample that is under investigation. This sample then refract the light or emits new photons and electrons that tell us something about how the structure of the sample.

As all the equipment along the long vacuum tube affects the experiment all parts of it need to be tuned with respect to the other parts to create the perfect conditions for examining the sample. Therefore, a control system that provides overview and possibility to tune the beam is needed. The control system also needs to be so easy to use that visiting scientists can finish all their necessary measurements within a few days without too much struggling with the beamline itself and without any risk of harming someone or destroying the equipment. Since the radiation has an extremely high intensity and easily can overheat, it can destroy parts that costs millions in seconds, or created harmful radiation that must not reach anyone working around the beamline.

While planning MAX IV, we decided to unify the control systems on the facility, to make it easier to deploy and maintain in the future. The work with selecting different components for the control system already started at the old facility (MAXLAB) where the soft X-ray beamline Species, Fig. 1, was built to serve as a test bench.

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In Fig. 1 the optical elements of Species, i.e. the mirrors, the grating and the baffles to screen the light, are shown. To be able to move those optical elements, 2-phase stepper motors are used on 56 individual axis run by IcePAP motion controllers, Fig. 2.

 

Now, the very vast majority of the 1500 motion axes at MAX IV, soon 2500, are using IcePAP. Some motors are moving plates 10 cm in or out of the beam, some have nanometer precision. Some motors move a sample with the size of a grain of salt, yet others move tons. Many motors need to be controlled as a group to perform complex and synchronized movements.

The motion controllers are arranged in cabinets where racks are stacked and joined through a CAN bus as in Fig. 2. Each rack can host eight drivers where each driver controls one motor and up to three encoders to tell the motor position. The first rack also has one master controller, while the rest of the racks have one slave controller each. The master is connected to the control system over Ethernet, as Ethernet is the back bone communication system of the entire control system.

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Figure 2. IcePAP motion controllers developed at ESRF are used to control 2-phase stepper motors. MAX IV has joined the IcePAP collaboration and use IcePAP as the default driver for all motions.

 

The control system software at MAX IV is built in Tango and uses the Python-based Sardana framework and runs on CentOS. The Tango/Sardana selection is not only made for the motion control part but for all the systems in need of control, such as vacuum, power supplies, valves, and cameras. Tango, through the Taurus module, also provides the environment in which the GUIs are created. Both Tango and Sardana are open source software and, since they have been used at other synchrotron facilities, code and experience have been accumulated over time. To deepen the transfer of knowledge and to contribute to the community, MAX IV is a part of both the Tango collaboration and the Sardana collaboration.

A synoptic, Fig. 3, is the first part of the control system visiting scientists gets acquainted to. The synoptic aid the scientists to get an overview of the beamline as well as hide functionality that short term visitors do not need to access.

From the synoptic, graphical user interfaces, GUIs, can be open to move the optics while collecting measurement data. The user tells the control system the conditions that is needed for an experiment, i.e. a particular energy of the beam, and the control system then moves several motors simultaneously to get to the required state.

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Figure 3. Synoptic overviews will aid the user to get an overview of the beamline.

 

To efficiently build a large number of new beamlines, a unified reference system, Fig.4, is also important, that defines the direction of motion. This helps the engineers and the scientist to talk the same language when describing issues and needs.

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Figure 4. MAX IV Beamline coordinate system.

A number of strategic decisions, such as the selection of one control system, the selection of one motor system, the selection of one computer operating system, the selection of one main programing language, and a number of other strategic selections helps making the facility transparent for the people working at Max IV and will, in the future, make the facility easier to operate and maintain for the forthcoming decades.

 

Peter Sjöblom Ph.D.
Research Engineer MAX IV
+46 702 999 397